Carboxylic acids react to form a number of important derivatives. Some types of compounds we can derive are:

Esters: Formed by reaction with alcohols. The hydroxyl (-OH) group on the acid leaves the molecule and joins up with the H atom from the alcohol's hydroxyl group to create water. The acid's -OH group is replaced by the O atom from the alcohol and the two compounds are united. For example, methanoic acid (formic acid) reacts with ethanol to form ethyl methanoate. To get back the original acid and alcohol, reflux with water in acid or alkaline conditions. Esters often have sweet smells and are used in colouring and flavouring - for example, methyl butanoate is found in pineapples.

Acyl chlorides (acid chlorides): Formed by reaction of carboxylic acids with chlorides, e.g. phosphorous pentachloride. The hydroxyl group is replaced by a chlorine atom, so for example ethanoic acid (acetic acid, CH3-COOH) becomes ethanoyl chloride (CH3-COCl). Due to the imbalance in the electronegativities of chlorine and carbon (chlorine is much better at attracting electrons), acyl chlorides are highly reactive. They will react with water to give the parent acid, with alcohols to form an ester, and with amines and ammonia to form amides. Put a drop or two into some water and get a lot of effervesence followed by choking hydrogen chloride fumes.

Amides: Formed by reaction with amines and ammonia. The hydroxyl group is replaced by -NH from the amine, or -NH2 from ammonia, to eliminate water and combine the two compounds.

Acid anhydrides: Formed when two carboxylic acid molecules condense (releasing water) to form an oxygen-rich compound such as ethanoic anhydride (C4H6O3).